Heart experts say a study which shows a lack of health benefits from a low-fat diet failed to take into account other risk factors.
Part of the study looked at breast cancer incidence
A US study of over 19,500 women found cutting back on fat for almost a decade failed to offer significant protection against some cancers and heart disease.
But British experts say salt levels were not cut, and many of the women were overweight or obese.
The study is published in the Journal of the American Medical Association.
Researchers running the Women's Health Initiative study of post-menopausal health asked more than 19,500 women aged 50 to 79 to reduce their total fat consumption and eat more fruit and vegetables.
The women were then monitored over eight years during the 1990s, and compared with another group of 29,300 women whose diet was unchanged.
Rates of breast and bowel cancer and heart disease were measured.
Although some improvements were seen in the study group, most were insignificant.
A 9% reduction in breast cancer incidence meant that out of 10,000 women, 42 in the low-fat group and 45 in the control group developed the disease each year.
And overall rates of bowel cancer were not lowered by the healthier diet - although reducing fat consumption did produce a 9% decrease in colon polyps which can lead to colon cancer.
In addition, no reduction was seen in heart disease risk among the women eating less fat.
Their rates of heart disease fell by just 3%, while blood levels of "bad" cholesterol, low density lipoprotein (LDL), dropped by only 2.4%.
The American researchers say that they are not disheartened by the results.
They said the difference between "good" and "bad" fats was not recognised when the study started, so women were only told to reduce fat consumption - rather than to reduce levels of trans-fats, which are classed as bad, while fats in nuts, fish and vegetable oils are said to have health benefits.
Dr Ross Prentice, one of the study authors, from the Fred Hutchinson Cancer Research Centre in Seattle, Washington, said: "While the study didn't give us the results that some people were hoping for, it suggests that we're on the right track.
"Women can be confident that cutting back on fat and following the recommended dietary guidelines... certainly won't hurt when it comes to maintaining a healthy lifestyle and preventing chronic disease."
But Judy O'Sullivan, a cardiac nurse at the British Heart Foundation, said: "Numerous studies have confirmed there are huge heart benefits from maintaining a healthy lifestyle which involves a balanced diet and regular physical activity.
"It is easy to identify a number of important reasons why this study did not agree with previous research.
"It didn't reflect current advice for good heart health, such as salt reduction, increasing intake of good fats such as those in oily fish, and increasing exercise.
"Additionally, most of the women in the study were overweight or obese, which increases your risk of developing diabetes - another risk factor for heart disease."
Dr Emma Knight, science information manager at Cancer Research UK, said: "People should continue to eat a healthy balanced diet, with lots of fruit and veg.
"At present, the only diet-related factors that definitely increase breast cancer risk are obesity and alcohol.
"Bowel cancer risk can be increased by a diet high in red and processed meat, but is lowered by eating a high-fibre diet."